Allow docker/dockerfile frontend
docker/dockerfile is accepted as a valid frontend in addition to the blubber/buildkit image.
Note that docker/dockerfile cannot do everything that blubber/buildkit can do. For example, it does not support --opt run_variant=true, so the kokkuri:build-and-run-image job template will not run the image.
To use the dockerfile frontend:
Set the BUILD_CONFIG variable to name of the Dockerfile (relative to BUILD_CONTEXT, which is usually the root of the project checkout), and set BUILD_VARIANT to either a blank string (to build the default target) or a string naming one of the "FROM" entries in the Dockerfile.
Bug: T326569